Curing rate is an important vulcanization parameter, since it, in part, determines the time needed for the compound to cure (for example, t90). In the manufacture of vulcanized rubber articles, significant cost saving can be realized through a reduction of cure time. By using secondary accelerator (ultra accelerator), the rate of cure increased, the cure time required to meet minimum states of cure can be reduced. However, the loading of secondary accelerator lead to premature crosslinking (low ts2) during the compounding or processing stage of the rubber formulation. The ethylene/propylene/diene (EPDM) polymers that contain low levels of diene, usually require very active cure compositions to reach high curing rates in certain applications, such as sponge weather strip. An active curing composition usually includes primary and secondary accelerators, which can lead to premature crosslinking during polymer compounding and processing.
It is desired that a rubber formulation maintain chemical stability during processing of the formulation, such that minimal vulcanization occurs prior to formation of the finished cured article. Theses compositions are especially needed in applications that require high cure rates, such for use in sponge weather strips.
U.S. Pat. No. 5,187,239 is directed to a sulfur vulcanizable rubber composition comprising an elastomer containing olefinic unsaturation, a vulcanizing agent, an accelerator, and from about 0.05 phr to about 5.0 phr of a methyl trialkyl ammonium salt. However, the shorter alky group, such as methyl group, in the ammonium salt decrease its acceleration on the curing rate, due to its relative lower transferability of curing agent into the rubber. Furthermore, the use of both primary and secondary accelerators to accelerate the curing rate results in poor anti-scorch property during compounding.
U.S. Pat. No. 4,861,842 is directed to a cure system for a sulfur vulcanizable rubber. The cure system comprises the following: a sulfenamide accelerator, a guanidine accelerator, a thiuram accelerator, a mercapto benzothiazyl disulfide, a sulfur vulcanizing agent, an N-cyclohexylthio-phthalimide, a zinc compound, and a methyltrialkyl ammonium salt. Again, the shorter alky group, such as methyl group, in the ammonium salt decreases its acceleration on the curing rate, due to its relative lower transferability of curing agent into the rubber. Furthermore, the use of both primary and secondary accelerators to accelerate the curing rate results in poor anti-scorch property during compounding.
U.S. Publication No. 2002/0180078 is directed to a rubber composition, which is free of precursor of at least one carcinogenic nitrosamine, and which contains at least one rubber selected from natural rubber, polyisoprene, polybutadiene, styrene-butadiene, styrene-isoprene and butadiene-isoprene copolymers, or styrene-butadiene-isoprene terpolymers. The composition comprises a vulcanization system containing the following: a) sulfur, b) at least one accelerator compound selected from the group consisting of benzothiazyl disulfide and mercaptobenzothiazole, c) at least one ultra-accelerator compound selected from the group consisting of tetrabenzylthiuram disulfide and zinc dibenzyldithiocarbamate; d) at least one vulcanization amine activator selected from the group consisting of amines, guanidines, aldehyde and amine condensates, and quaternary ammonium salts. Furthermore, the use of both primary and secondary accelerators to accelerate the curing rate results in poor anti-scorch property during compounding.
European Patent Application No. 0982355A1 is directed to rubber formulations containing an accelerator compound, comprised of an organic-based quaternary ammonium salt. Preferably, the accelerator compound is a quaternary ammonium salt selected from the group consisting of trimethylbetahydroxyethyl ammonium hydroxide (TMBHEAH), carboxymethyltrimethyl ammonium hydroxide (CMTAH), methylallylbenzylphenol ammonium iodide, benzyltriethyl ammonium chloride, hexadecyltrimethyl ammonium chloride imidazolium compounds. More preferred accelerators are comprised of a choline based compound, comprised of trimethylbetahydroxyethyl ammonium hydroxide and/or of a betaine compound, comprised of carboxymethyltrimethyl ammonium hydroxide.
Quaternary ammonium salts have been used as additives for rubber composites. For example, as a dispersing agent for fillers, such as silica, and as modifiers for nano clay.
U.S. Pat. No. 6,025,428 is directed to a method of compounding silica in a tire tread composition, comprising the following steps: (a) mechanically mixing, in a plurality of mixing stages the following: a rubber comprising a conjugated diene, a silica dispersing agent for enhancing the distribution of the silica in the rubber, and a coupling agent for chemical bonding of the rubber to the silica; and (b) mixing the rubber and the silica with the coupling agent, prior to, or contemporaneously with, mixing the dispersing agent the rubber. The dispersing agent comprising a quaternary ammonium compound.
Japanese Patent Application No. P2005-247962A is directed to a silica-compounded rubber formulation comprising at least one diene rubber, silica, and an organic ammonium compound containing at least one allyl or benzyl group.
Japanese Patent Application No. 2006-290940 is directed to a tread rubber composition, comprising a rubber component, a filler, and an onium salt. The invention is disclosed as providing a tire tread rubber composition for reduced fuel consumption, which lowers the tan δ at 60° C., without substantial detriment to other properties.
Japanese Patent Application No. H9-316238 is directed to a composition for compounding a rubber, comprising an organic phosphonium salt or an organic ammonium salt, and one or more fillers. The salt can be easily, uniformly kneaded when it is added into the rubber.
U.S. Pat. No. 2,531,396 is directed to a reinforced elastomer, comprising, an elastomer base, selected from a group consisting of natural rubber, butadiene rubber copolymers and polychloroprene rubbers; and a modified clay, originally exhibiting a base-exchange capacity from 10 to 100, in which the inorganic cation has been replaced by a substituted organic onium base to the extent of at least 15 me/100 g of the clay, or replaced by a substituted organic ammonium base to the extent of at least 15 me/100 g. of clay.
U.S. Pat. No. 6,858,665 is directed to a process of preparing a rubber composition, which contains a dispersion of intercalated and at least partially an exfoliated smectite clay. The smectite clay is intercalated and exfoliated, in situ, within an elastomer host. The process comprises blending the following at least one hydrocarbon diene-based elastomer; a smectite clay selected from at least one of montmorillonite clay and hectorite clay and mixtures thereof; a quaternary ammonium salt selected from methyl trialkyl ammonium chloride, trimethyl alkyl ammonium chloride, dimethyl dialkyl ammonium chloride, dimethyl alkyl allyl ammonium chloride and dimethyl diallyl ammonium chloride; and at least one additional reinforcing filler comprised of at least one of carbon black, synthetic amorphous silica or silica treated carbon black and mixtures thereof. A coupling agent is mixed, therewith, subsequent to said intercalation of the smectite clay, and after at least a partial exfoliation of the intercalated clay to form exfoliated clay platelets.
Additional rubber formulations are disclosed in International Publication No. WO 2004/052983.
As discussed above, there remains a need for improved rubber formulations which have enhanced cure rates at specified cure temperatures, and maintain stability (scorch resistance) at processing temperatures. There is a need for cure compositions that provide high cure rates, while minimizing premature crosslinking under standard processing temperatures, and which can be used in applications that require high cure rates. These needs and others have been met by the following invention.